Recommended limit_req and limit_conn for location ~ \.php$ {}

[itpp2012]

General rule of thumb is set it as low as possible, as soon as 503's are
getting your users upset or resources are getting blocked, then double the
values, keep an eye on the logs, double it one more time when required,
done.

Posted at Nginx Forum: https://forum.nginx.org/read.php?2,269862,269866#msg-269866